In a constant-mesh type gear transmission provided with a transmission clutch in a transmission system for a vehicle, one gear position is established through a process in which a shifter supported on a transmission shaft so as to be movable in the axial direction of the shaft engages with a transmission gear rotatably supported on the transmission shaft to thereby transmit power.
Shifting of gear position is performed through a process in which a transmission gear and a shifter for a current gear position are disengaged from each other and a transmission gear and a shifter for a next gear position are engaged with each other. In this case, the engagement between the transmission gear to which power has been transmitted and the shifter is so strong that a large resistance against disengagement is present because of friction between engaging portions of the transmission gear and the shifter that are pressed against each other. Ordinarily, therefore, a transmission clutch is disengaged to interrupt transmission of power, and, in this condition, the transmission gear and the shifter are disengaged from each other without resistance, whereby the gear position can be smoothly shifted (see, for example, Patent Document 1).
In addition, in the case of a transmission system including two transmission clutches (see, for example, Patent Document 2), shifting of gear position is conducted as follows. While a shifter is in engagement with a transmission gear of a current gear position, a shifter is engaged with a transmission gear of a next gear position, to establish a state in which the transmission gears of the current gear position and the next gear position are both in engagement with the shifters. In this state, the transmission clutch on one side that has been contributing to transmission of power to the transmission gear of the current gear position is disengaged, and the transmission clutch on the other side is engaged, to thereby switch over the power transmission to the next gear position. In this way, the shifter is easily disengaged from the transmission gear of the current gear position, which transmission gear is free from power transmission because of the disengaged associated transmission clutch. Thus, the next gear position is easily established.